1. Field of the Invention
The present invention relates to a light emitting diode and, in particular, to a high-power semiconductor light emitting diode (power LED).
2. Related Background Art
Semiconductor light emitting diodes are elements that use spontaneous emission of light that occurs in the course of recombination of injected carriers with holes in a region of a PN junction region when a forward current is supplied to the PN junction. Small-sized LEDs whose chip size does not exceed 300 μm in width and depth have heretofore used frequently as semiconductor light emitting diodes. This conventional structure LEDs has advantages of low power consumption, long lifetime, compactness and lightweight, etc., and they are widely used in various kinds of display devices and traffic lights. Especially in recent years, high luminance emission under a low current (around 20 mA) is requested for use as backlights of automobiles.
In general, semiconductor light emitting diodes can emit higher luminance light as the internal emission efficiency and the light extraction efficiency become higher and higher, respectively, the internal light emission efficiency representing the ratio of the radiative carrier recombination relative to the carrier recombination (radiative carrier recombination and non-radiative carrier recombination), and the light extraction efficiency representing the ratio of extracted light relative to light generated by the radiative carrier recombination. As being capable of obtaining large internal light emission efficiency, a structure using an InGaAlP compound semiconductor of a direct transition type as its active layer has been known. But, the InGaAlP compound semiconductor is formed on an opaque GaAs substrate. Under the circumstances, as a structure ensuring large external light extraction efficiency, a compact LED of a transparent substrate type has been brought into practical use, which is made by first making an InGaAlP compound semiconductor by crystal growth on a GaAs substrate, subsequently bonding a transparent GaP substrate and removing the opaque GaAs substrate. A conventional structure LED of this type is proposed, for example, in JP2001-57441A.
Recently, development of power LED is under progress. This power LED is a large-sized, high-power LED having an area of the top surface of the chip as large as 0.1 mm2 or more. Its package is reduced in heat resistance, and a large current even beyond 50 mA can be supplied. This power LED is expected for its use as a substitution of light bulbs or in industrial machines, analytical instruments, medial apparatuses, and so on. Also, as such power LEDs, those of a transparent substrate type using an InGaAlP compound semiconductor as the active layer and bonding a transparent GaP substrate have been brought into practical use.
FIGS. 8 and 9 show a conventional power LED of the above-mentioned transparent substrate type. FIG. 8 is a cross-sectional view thereof, and FIG. 9 is a plan view. The substrate 501 has a width and a depth of approximately 550 μm, and the area of its inner surface A is approximately 0.3 mm2. This is a large-sized LED. Sequentially formed on the transparent p-type GaP substrate 501 are: a p-type GaP bond layer 502; p-type InGaP bond layer 503; p-type clad layer 504 of InAlP; active layer 505 having a MQW structure including p-type InGaAlP; n-type clad layer 506 of InAlP; current diffusion layer 507 of n-type InGaAlP; and n-type contact layer 508 of GaAs. The p-type GaP bond layer 502 and the p-type InGaP bond layer 503 are formed by bonding, and a bonded interface is formed between them. A p-side ohmic electrode 510 is formed as one of electrodes under the p-type GaP substrate 501 when viewed in the figure. An n-side ohmic electrode 511 as the other of the electrodes is formed on the top when viewed in the figure. As to the actual thickness of this LED, thickness of the transparent electrode 501 (including the p-type GaP bond layer 50) is several hundreds of μm, and thickness of the epitaxial growth layers 503 through 508 is some μm. However, FIG. 8 illustrates it in a modified scale for easier explanation.
In the power LED shown in FIGS. 8 and 9, light generated in the active layer 505 is externally emitted from the top surface or side surface of the diode when viewed in the figure. In order to extract the light efficiently from the top surface, the opaque n-side ohmic electrode 511 is formed to occupy an area as small as shown in FIG. 9. The power LED of the transparent substrate type formed in this manner is enhanced in external light extraction efficiency and can emit light of higher luminance than diodes formed on opaque GaAs substrates.
A power LED higher in light extraction efficiency than the conventional power LED, if any, will be effectively useful for various purposes, such as the use as a substitution of light bulbs as mentioned above, for example. However, the power LED is scheduled for use under a high current, reduction in operation voltage is extremely important. Heretofore, it has been the general recognition that enhancement of the external light extraction efficiency without inviting an increase of the operation voltage is usually difficult. Consequently, it has been considered extremely difficult to enhance the light extraction efficiency of the conventional power LED further more.
That is, the effort of enhancing the light extraction efficiency in compact conventional structure LEDs heretofore relied on diminishing the electrode or etching the diode to an appropriate configuration. However, this approach by diminishing the electrode or etching the diode may cause an increase of the operation voltage. Conventional structure LEDs, however, are used under a low current, and such an increase of the operation voltage has not been recognized as a serious issue. In contrast, unlike such conventional structure LEDs, power LEDs are scheduled for use under a high current. Therefore, it is quite important for power LEDs to keep a low operation voltage from the viewpoint of the power consumption, reliability, lifetime, and the like. However, it has been believed extremely difficult practically to enhance the luminance without inviting an increase of the operation voltage. Consequently, it has been believed extremely difficult to enhance the light extraction efficiency of power LEDs further more.